Silicone compositions which are cured to form silicone rubbers are well known. Because of the excellent properties of the cured product, such as excellent weather- resistance, heat-resistance, cold-resistance and electric insulating property thereof, the composition has widely been used as various shaping materials, such as potting material, coating material or framing material, for electric and electronic parts. As the case may be, a silicone composition which is naturally an insulating material may be made electroconductive to give an electroconductive silicone composition.
Methods of making a silicone composition electroconductive by incorporating an electroconductive carbon black into the composition are generally known. For instance, JP-A-54-139659 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an electroconductive organopolysiloxane elastomer containing both a furnace black and an acetylene black; JP-A-55-120656 discloses a liquid organopolysiloxane composition containing hollow shell-shaped carbon black grains having a surface area of 900 m.sup.2 /g or more; JP-A-55-108455 discloses an extrudable liquid organopolysiloxane composition containing both a carbon black and an electroconductive fibrous material; and JP-A-56-120761 discloses a silicone rubber composition containing a carbon black having a specific surface area of 80 m.sup.2 /g or more .
Carbon black, which is incorporated into a silicone composition for the purpose of making the composition electroconductive, may be classified into various kinds of thermal black, furnace black, acetylene black, etc., in accordance with the means of manufacturing the same. Every type of carbon black contains a slight amount of sulfur.
It is well known that the sulfur in carbon black will be a factor in inhibiting the crosslinking reaction of a curing silicone composition. Accordingly, in the case of a silicone rubber composition containing a carbon black as a filler, a large amount of a crosslinking agent is necessary for elevating the curing speed of the composition. Further, since a carbon black has a high surface activity, it adsorbs the components of a crosslinking agent. In the situation where the amount of the carbon black in the silicone composition is increased, it would be difficult to obtain the necessary crosslinking density merely by increasing the amount of the crosslinking agent. Additionally, there is still another problem in that an increase in the amount of the crosslinking agent in the silicone composition lowers the rubber characteristics of the hardened silicone rubber derived from the composition.